The KBS Tuff is an ash layer in the Koobi Fora Formation east of Lake
Turkana in northern Kenya. It is significant because hominid fossils and
artifacts were found in and under it, so its age gives a minimum age of
the fossils. Various attempts to date it have yielded a wide range of
different results, from 0.52 to 220 million years. The dating of the KBS
Tuff exposes the fallacies of radiometric dating. "Good" dates are chosen
to accord with accepted dates of fossils, while anomalous dates may not be
reported at all. And in practice, it is impossible to be sure one has
selected uncontaminated samples.
The KBS Tuff controversy illustrates many of the problems with
radiometric dating, but it equally illustrates that the problems are
not insurmountable.
The KBS Tuff (for "Kay Behrensmeyer Site," after the geologist who
first described it) is a layer of redeposited volcanic ash, so it
contains a mixture of older sediments, too. It is still possible to
date the layer, but care must be taken to choose only the youngest
rocks, else one would be dating the age of older sediments washed into
the layer, not the age of the layer itself. This is what happened with
the first ages reported from the tuff. In a study to test the
feasibility of dating samples from the tuff, the samples were
contaminated with non-juvenile components which could not be separated
out, giving ages over 200 million years. It was recommended that new
samples be collected from which suitable individual crystals could be
separated (Fitch and Miller 1970). These new samples were dated at
2.61 +/- 0.26 million years, based on the
40Ar/39Ar dating method (Fitch and Miller 1970).
Discrepancies with this date soon turned up, though. Work with animal
fossils, particularly of pigs, showed that the strata in question
matched younger strata in the nearby Omo Valley. In its early stages,
this fossil work was imprecise enough that the 2.61 Myr date could
still be justified (Maglio 1972). However, the fossils continued to
point to a younger date as the quality of the work on them improved
(White and Harris 1977). And in 1975, another lab, using K-Ar dating,
reported dates of 1.82 and 1.60 Myr (Curtis et al. 1975).
Fitch and Miller turned to an independent method to resolve the
discrepancy, fission-track dating. Initial results gave an age of 2.44
+/- 0.08 Myr (Hurford et al. 1976). This fit well with the age of 2.42
Myr, which Fitch et al. (1976) recalculated from their original
results. Subsequent 40Ar/39Ar measurements they
took gave a scattering of ages from 0.52 +/- 0.33 to 2.6 +/- 0.3 Myr.
They attributed the spread to reheating of the crystals after
deposition. Paleomagnetic studies gave ambiguous results (Brock and
Isaac 1974; Hillhouse et al. 1977).
The weight of evidence soon began to converge on an age near 1.9 Myr,
though. A study of trace elements in the minerals showed that the KBS
Tuff correlates with the H2 tuff in the Shungura Formation,
uncontroversially dated about 1.8 Myr (Cerling et al. 1979). The 1.60
Myr age reported by Curtis et al. (1975) was found to be an error due
to a faulty balance (Drake et al. 1980). A later fission-track study
which took pains to eliminate possible errors gave an age of 1.87 +/-
0.04 Myr (Gleadow 1980). Because the controversy had become quite
heated, another expert, Ian McDougall, was called in to do independent
dating. He came up with an age of 1.89 +/- 0.01 using K-Ar dating and
1.88 +/- 0.02 using 40Ar/39Ar dating (McDougall
et al. 1980; McDougall 1981, 1985). Geological evidence and the
consistency of dates derived from various sources indicates that
reheating after deposition is unlikely.
The lessons to be learned from the KBS Tuff dating controversy are not
that radiometric dating does not work, but that it works with some
caveats.
Some formations are easier to date than others. The KBS Tuff was
particularly difficult to date because it included volcanic
sediments of several different ages. Furthermore, it looked the
same as other tuffs, so care was needed to make sure the same layer
was being referred to in different areas. All of this requires
careful work from knowledgeable geologists. Were it not for its
importance to determining the ages of important hominid fossils,
geologists probably would not have bothered with dating it at all.
Some dating techniques are simply inappropriate in some
circumstances. As noted above, paleomagnetic study is not
particularly useful at this site.
Discrepant dates are not dismissed out of hand. In addition to
trying to resolve the issue with further dating, the discrepancies
caused people to look for the sources of error. The original
erroneous date by Fitch and Miller could be an accurate date of a
roughly 2.5 Myr ash layer, present in neighboring areas but
apparently eroded from the Koobi Fora Formation. Apparently, some
pumice from that volcanic event had been incorporated into the KBS
Tuff. Samples sent to an independent lab for "blind" dating
confirmed its older age (Fitch et al. 1996). Alternatively, this
and other discrepant ages may be due to contamination with older
material. Such contamination caused ages in the 2.0 - 6.2 Myr range
in the analysis of Curtis et al. (1975) until they revised their
sample purification procedures. A high atmospheric argon
contamination in their samples and analytical errors may have
contributed, too (McDougall et al. 1980).
The fission-track study which gave the 2.44 Myr age was the first
such study to date zircons so young. The reanalysis by Gleadow
(1980) noted problems with the standard methods and contributed new
methodology for dealing with zircons with low track densities.
People's preconceptions and personalities can get in the way of
evaluating the data objectively. In the KBS Tuff controversy,
personality conflicts may have contributed to delay in the
resolution and certainly contributed to the drama. But in the end,
the objective evidence is a constraint that every scientist must
meet. Replication, free access to information, and awareness of
conflicts of interest help assure that personal foibles do not
determine outcomes. Because such mechanisms were in place, all of
the scientists who initially supported the older 2.6 Myr date for
the KBS Tuff later came to accept the 1.88 Myr age (Lewin 1987).
Note that different methods give the same results when known sources of
error are removed. K-Ar, 40Ar/39Ar, and
fission-track methods ultimately all gave the same results. These
results were correlated with strata of the same age at other locations
on the basis of fossil and trace element analysis.
The different ages which were seriously debated for the KBS Tuff, from
1.6 to 2.6 million years, were never close to ages required by
young-earth creationism.
Brock, A. and G. Ll. Isaac. 1974. Paleomagnetic stratigraphy and
chronology of hominid-bearing sediments east of Lake Rudolf, Kenya.
Nature 247: 344-348.
Cerling, T. E., F. H. Brown, B. W. Cerling, G. H. Curtis and
R. E. Drake, 1979. Preliminary correlations between the Koobi Fora and
Shungura Formations, East Africa. Nature 279: 118-121.
Curtis et al. 1975. Age of KBS Tuff in Koobi Fora Formation, East
Rudolph, Kenya. Nature 258: 395-398.
Drake, R. E., G. H. Curtis, T. E. Cerling, B. W. Cerling and J. Hampel,
1980. KBS Tuff dating and geochronology of tuffaceous sediments in the
Koobi Fora and Shungura Formations, East Africa. Nature 283:
368-372.
Fitch, F. J. and J. A. Miller. 1970. Radioisotopic age determinations
of Lake Rudolf artefact site. Nature 226: 226-228.
Fitch, F. J., P. J. Hooker and J. A. Miller. 1976.
40Ar/39Ar dating of the KBS Tuff in Koobi Fora
Formation, East Rudolf, Kenya. Nature 263: 740-744.
Fitch, F. J., J. A. Miller and J. G. Mitchell. 1996. Dating of the KBS
Tuff and Homo rudolfensis. Journal of Human Evolution 30:
277-286.
Gleadow, A. J. W. 1980. Fission track age of the KBS Tuff and
associated hominid remains in northern Kenya. Nature 284: 225-230.
Hillhouse, J. W., J. W. M. Ndombi, A. Cox and A. Brock. 1977.
Additional results on paleomagnetic stratigraphy of the Koobi Fora
Formation, east of Lake Turkana (Lake Rudolf), Kenya. Nature 265:
411-415.
Hurford, A. J., A. J. W. Gleadow and C. W. Naeser. 1976. Fission-track
dating of pumice from the KBS Tuff, East Rudolf, Kenya. Nature 263:
738-740.
Lewin, Roger. 1987. Bones of Contention. New York: Simon and
Schuster.
Maglio, Vincent J. 1972. Vertebrate faunas and chronology of
hominid-bearing sediments east of Lake Rudolf, Kenya. Nature 239:
379-385.
McDougall, Ian, Robyn Maier, P. Sutherland-Hawkes and A. J. W. Gleadow.
1980. K-Ar age estimate for the KBS Tuff, East Turkana, Kenya.
Nature 284: 230-234.
McDougall, Ian. 1981. 40Ar/39Ar age spectra from
the KBS Tuff, Koobi Fora Formation. Nature 294: 120-124.
McDouball, Ian. 1985. K-Ar and 40Ar/39Ar dating
of the hominid-bearing Pliocene-Pleistocene sequence at Koobi Fora,
Lake Turkana, northern Kenya. Geological Society of America
Bulletin
96: 159-175.
White, T. D. and J. M. Harris. 1977. Suid evolution and correlations
of African hominid localities. Science 198: 13-21.
Further Reading:
Hay, R. L. 1980. The KBS tuff controversy may be ended. Nature 284:
401.
Johanson, Donald and James Shreeve. 1989. Lucy's Child, New York:
Avon,
pp. 91-101.
Lewin, Roger. 1987. Bones of Contention. New York: Simon and
Schuster,
chaps. 9-10.